I use it and there is no problem. Slackel uses Firefox-57 now in repos because of Meltdown and Spectre.
I suggest to move ~/.mozilla folder to ~/.mozilla-bak and then run Firefox to create a new profile. Then copy the key3.db, logins.json and places.sqlite files from ~/.mozilla-bak/firefox/something.default/ to new created ~/.mozilla/firefox/something.default/ to have saved logins and passwords and bookmarks you may have saved.

Thanks!
What I did, made a back-up as suggested. Then copied the
entire something.default over to the new ~/.mozilla/firefox
and changed profile.ini to the backed-up something.default.
That gave me better results and I am using now Firefox-57.

Firefox versions <=56 or so had issues, and Firefox ESR was definitely a better choice. But this is no longer the case. Slackel uses Firefox Quantum (version 59.0.2 right now), and I can tell not only it works flawlessly, but it is also considerably faster/less resource-demanding than Firefox ESR. The difference is noticeable even on fast computers with lots of resources, and it is blatantly apparent on old computers.

As mimosa already pointed out, you can manually download and use Firefox Quantum on Salix. I just extracted the tarball at home directory and created a desktop shortcut, but you can also add it to your PATH. Note that you don't have to uninstall Firefox ESR - and you probably shouldn't: Firefox Quantum will use your addons already installed at Firefox ESR. They basically share the same user folder at home directory.

A pleasant detail in this forum: several people pick a picture of their pet as their avatar. Who am I to do otherwise?